Sanitary Flow Rate for a Church?? 1

[MichaelMiller]

I have to design a small lift station for a church in Georgia. Does anyone know what kind of flowrate I should use to size the pump?

If I go with the Georgia Septic Manual Flow of 7 GPD/Seat, I get 6300 GPD for a 900 seat sanctuary; which is 4.4 GPM. With a peaking factor of 4, that gives me 17.6 GPM. That seems a little low.

If I assume a 4 hour day instead of 24 hour day, I get 26.25 GPM. With a peaking factor of 4, that gives me 105 GPM.

Flow Based on Fixture count is 135 GPM if everything is running at one time.

I am thinking I'll use the 105 GPM for the pump but I hate to oversize the pump and have the church pay more. Any experience/feedback will be appreciated.

 

[LHA]

Per Pennsylvania Code changes effective through 35 Pa.B. 1322 (February 12, 2005). Title 25, Environmental Protection, Chapter 73. Standards for Onlot Sewage Treatment Facilities, § 73.17.Sewage Flows. Source:

http://www.pacode.com/secure/data/025/chapter73/s73.17.html

Churches (per seat) 3 gpd
Churches (additional kitchen waste per meal served) 3 gpd
Churches (additional with paper service per meal served) 1.5 gpd

So, your 7 gpd/seat looks good, but could be reduced.

PF = 4 is OK for such an irregular use, with a 24 hour day. OR use the 4 hour day, with a lower PF of around 2.

By using the peak hours AND the high PF for the pump size, you are counting the effects of the peak flow twice.

All fixtures running at once is not realistic, and too conservative.

I would design for the pump and wetwell for the gpd with respect to food service, at PF=2, over 4-6 hours.

Remember: The Chinese ideogram for “crisis” is comprised of the characters for “danger” and “opportunity.”
-Steve

 

[bimr]

The minimum size of the force main is going to set the pumping rate.

The minimum size for a sanitary wastewater force main is usually 4" dia. The minimum flow velocity in a sanitary force main is 2 feet per second at design rate. That would calculate into a design average pumping rate of 80-90 gpm. The maximum flow rate will be set by the pump curve, but will probably end up being 20% higher.

The peak flow out of the church that you have calculated (17.6 GPM) is acceptable, but the lift station needs to be operated at a higher capacity to maintain the minimum flow.

 

[dicksewerrat]

What is supplying the water for the church? Can you get 17 GPM? Maybe the flow rate should be the supply number. And the wetwell should be large enough to have pump run 6-10 minutes per turn on.

 

[bimr]

It is a correct assumption that you are discussing a lift station rather than a building sanitary ejector sump pump?

Another concern to address is that the sewage is going to septic in the lift station during the off hours. You should consider minimizing the wet well filling time by reducing the size of the wet well.

Also, consider the use of grinder pumps. The minimum size force main would then be 3" dia. with a flow of 70 gpm.

 

[francesca]

I agree with bimr that the minimum size of the forcemain is going to set your pump rate. I would also caution against using anything smaller than a 4" diameter main.

You can check your maximum flow against the design of the church itself: what size internal piping are they using and how many pipes discharge into the lift station? Come up with a practical maximum inflow to the lift station to check if your 4" scour velocity flow (with one pump running) will be able to keep up with the inflow.

Bearing in mind that the church will likely only be in use for a few hours only a few days a week, you may have an odour problem with the lift station, as the small volume between the bottom of the lift station and the pumps off elevation will go septic, as will any sewerage left in the forcemain.

I would design the lift station with sufficient capacity for normal operation, with the wet well sized with sufficient capacity to handle any peak events, like weddings or church fetes. Still, the maximum inflow will be controled by the size and number of lines into the lift station itself.

 

[MichaelMiller]

Thanks everyone for the valuable insight and recommendations. I believe we have designed a system that will work well for the church.

Since the church may develop further in the future and we have to extend the force main down the road through public and private property, I have decided to use a minimum 4" diameter pipe.

And @ 105 GPM, that lets me achieve a velocity of +/- 2.8 fps now and then in the future if they want to expand, the force main will be large enough to double flow without the velocity exceeding 6 fps. We sure want to put a large enough pipe in the ground now so the church won't have to go back later and upgrade the force main. Thanks Again!!